Process for molding ceramics

ABSTRACT

A wet molding method in which a ceramic slurry is charged into a cavity and uniaxially pressed by a punch to remove excess liquid from a portion of the slurry facing the punch to effect molding, the method being improved by maintaining at least one of the following conditions; 
     (a) the pressing of the slurry is stopped at a time between T and 1.5 T, T being defined as the pressing time necessary to remove sufficient excess liquid from the slurry in the mold to produce a molded mass; or 
     (b) the punch displacement position at a time at which sufficient excess liquid is removed from the slurry in the mold to produce a molded mass is less than 17% of the total mold length.

This is a Continuation-in-part of U.S. patent application Ser. No.08/109,369, filed Aug. 19, 1993now abandoned, which claimed priority ofJapanese Patent Applications 4-241874, filed Sep. 10, 1992 and 5-52364,filed Mar. 12, 1993.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a molding process for various ceramicproducts, particularly to a pressure slipcast molding of ceramic slurry,and a mold used therefore.

2. Description of the Prior Art

In a pressure slipcast molding process, a mixture consisting of powderand liquid (hereinafter “slurry”) is pressurized to discharge the liquidtherefrom. In this process, the higher the pressure is, the more denselythe powder is compacted. Thus, the resulting molded article has a highgeometric stability and the liquid discharge and from the slurry can bemade in reduced time. In the conventional pressure slipcast molding,porous molds made from gypsum or plastics have been used for the liquiddischarge, (e.g. Japanese Patent Publication No. 2-42321, JapanesePatent Laid-Open Nos. 60-70701, 63-3906 and 61-297103).

However, since these porous materials constituting the conventionalmolds do not have a sufficient strength, the molding pressure is, atmost only 10 kg/cm² when a gypsum mold is used and about 50 kg/cm² whena plastics mold is used. If the molding pressure exceeding this upperlimit is applied to the porous mold, such high pressure may bring aboutthe breakage of the molds. Thus, there is limitation in improving thedensity of ceramic molded articles and it becomes impossible to mold ina short time, depending on the shape or the size. In the method ofmolding ferrite slurry and so on, a filter cloth 9 and a filter paper 10are, as shown in FIG. 3, set at the frontal face of a metallic mold 3,which has holes of 1-3 mm inside diameter for dehydration. And onlyliquid (water) is discharged from the holes of the mold through thefilter.

In the method as shown in FIG. 3, since the liquid discharge partconsists of the filter cloth 9, filter paper 10 and mold 3 having theholes 4 for dehydration, a high molding pressure can be applied. Howeverin this case, when pressure is applied, the powder in the slurry entersthe holes for dehydration pressing on the filter paper or filter clothand then it becomes protrusions 11 are formed on the surface of themolded product, corresponding to the holes as shown in FIG. 4.Therefore, an after-treatment is necessary to remove the protrusion fromthe molded product, thereby resulting in a high production cost.

SUMMARY OF THE INVENTION

With the above in view, the present invention was made to overcome theabove and other problems encountered in the prior art.

It is an object of the present invention to provide a process forslipcast molding of ceramics, with a smooth surface in which a higherpressure is applied as compared with a conventional process.

Another object of the present invention is to provide molds used forcarrying out the above-mentioned molding process.

The basic idea of this invention is to enable the application of highpressure by using a mold made of a high-strength porous metal or ceramicmaterial in a pressure slipcast molding process of a ceramic slurry.

To the above and other ends, the present invention provides a processfor molding a ceramic slurry, in which a mold made wholly or partiallyof a porous metal or a porous ceramic is used.

The present invention also provides a mold made wholly or partially of aporous metal or a porous ceramic, for the purpose of molding ceramicslurry.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical cross-sectional view showing a casting methodemploying a casting mold according to an embodiment of the presentinvention.

FIG. 2 is a view similar to FIG. 1, showing a casting method employing acasting mold according to a modification of the present invention.

FIG. 3 is a view similar to FIG. 1 employing a conventional castingmold.

FIG. 4 is a vertical cross-sectional view showing a cast articleobtained using the conventional casting mold.

FIG. 5 is a view showing molded article in the form of circulartrumpets.

FIG. 6 is a vertical cross-sectional view showing a casting method formolded article in FIG. 5.

FIG. 7 is a graphical representation showing the relation betweenpressurizing time and punch position.

FIG. 8 is a view illustrating a pressurizing steps of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The basic concept of the present invention consists in that, in pressureslipcast molding of ceramics, a casting mold made of metal or ceramic ofa higher strength is employed to enable application of a higherpressure.

In case of using a material, whose main component is Fe, e.g. stainlesssteel, it becomes possible to develop a molding pressure of 100 kgf/cm².Even a pressure up to 1000 kgf/cm² can be developed depending on themold's design. Metallic materials are not limited to Fe metals, Cumetals, Al metals and so on.

Similarly, in case of using a hard material, e.g. Al₂O₃. A pressure notlower than 100 kgf/cm² may be developed. A pressure up to 1000 kgf/cm²is also possible depending on the mold's design. Ceramics materials arenot limited to Al₂O₃ ceramics, Si₃N₄ ceramics, BN ceramics and so on.

Also, it is possible to polish the surface of porous metals andceramics, to decrease their surface roughness, and to make it a mirrorplane in some cases. Thus, when the smoothed surface is used for a moldtouching the slurry, molded ceramic articles can be easily taken out ofthe mold and also be prevented from breaking. Furthermore, the surfaceappearance of the molded ceramic articles become excellent. In case ofusing ceramics for porous materials, ceramics have a high chemicalresistibility against acid, alkali and so on. Ceramics are neithercorroded nor rusted and have sufficient durability. And they areexcellent in strength and toughness, compared with other materials,therefore they do not become fatigued and have sufficient durabilitymechanically too.

The larger the diameters of pores in the porous material are, the moreeasily liquid in the slurry is discharged. But if the pores are toolarge, ceramic powders in the slurry are carried away. Therefore theupper limit of the pores' size should be determined according to thediameters of ceramic powders in the slurry.

In the above-described casting method by the liquid discharge from theslurry, it is necessary that diameters of pores in the porous materials,which are components of a mold the wholly or partially, should be notless than 0.1 μm so that dehydration and molding can be carried out inindustrially practical time. If diameters of the pore are less than 0.1μm, in case of using water for liquid of the slurry, the surface tensionbecomes higher, keeps the liquid from flowing into pores of porousmaterials, and makes it difficult to discharge the liquid in the slurrythrough pores of porous materials.

It has been found that powders in the pressurized slurry tend to undergocohesion to form agglomerates (hereinafter referred to as “secondaryparticles”). The maximum diameter of the pores is twenty times as highas the diameter of the secondary particles of powders in the slurry andit can prevent the powders from flowing.

Namely, if the powders flow into pores in porous materials from thesurface by the given pressure, the adherence between molded ceramicarticles and porous materials becomes intense, by an anchor effect, andmolded ceramic articles will be broken or chipped. When diameters ofpores in the porous materials are not more than twenty times as large asthe diameter of the secondary particles in the slurry, we have confirmedthat powders produce bridging and do not enter the inside of pores, andthat the molding can be done in a short time and mold release can beeasy.

When a surface roughness of the pore-free portion of porous metal orceramics, whose surface touches the slurry, is more than 0.4 μm Rz, theadherence between molded ceramic articles and porous materials becomesintense by an anchor effect. Particularly in case of applying highpressure, the adherence between molded ceramic articles and porousmaterials becomes intense because of the pressure applied to the partand it makes mold release difficult.

When a surface roughness is not more than 0.4 μm Rz, the adherence atthe above-mentioned portions becomes weak to enable the molded articlesto be released smoothly from the mold. Throughout the specification, allsurface roughness values are indicated in terms of the ten-point meanroughness Rz defined in Japanese Industrial Standard (JIS) B 0601.

In the invention, it is also possible to set filter paper and/or filtercloth on the surface of porous metal or ceramics facing the slurry. Evenif diameters of pores on the surface of porous metal or ceramics are solarge as to let the powders in the slurry enter the pores, the filterprevents powders from entering them and makes release easier. And if thesurface of porous metal or ceramics is rough, the filter does not bringabout an anchor effect and makes release smooth. Because the filter,made of paper and/or cloth has high flexibility, therefore it ispossible to release the filter from molded ceramic articles slowlywithout producing chips. Protrusions on the surface of ceramics, in theconventional method as shown in FIG. 3, do not enter pores of porousmetal or ceramics through the filter and then molded ceramic articleshave an excellent surface. In order to prevent powders in the slurryfrom passing through the filter and in order to get considerable speedfor excluding the liquid, it is desirable that the average diameter ofpores in the filter paper and/or filter cloth should be not less than0.1 μm and not more than twenty times as large as the diameter of thesecondary particles in the slurry. To discharge the liquid atsatisfactory speed for industrial needs, the average diameter of poresneeds to be more than 0.1 μm. If the average diameter is larger thantwenty times as large as the average diameter of the secondary powder inthe slurry, it causes efflux of powder, namely a permeation of theslurry through the filter for the same reason as discussed above. Thepaper filter may be formed of any material customarily employed for thepaper filter, while the cloth filter may be formed of any material, suchas synthetic fibers, e.g. polyester, nylon or acrylic fibers, or naturalfibers, such as cotton, provided that such material can be woven orknitted to form a cloth.

It is also possible that liquid in the slurry is sucked dry throughporous ceramics from the back and is discharged forcibly. In this case,filter paper and/or filter cloth could be placed on the surface ofporous metal or ceramics facing the slurry. Such forcible suction makesit possible to shorten a stiffening time of powders in the slurry and iseffective for a molded ceramic articles having an increased wallthickness.

With the pressure molding according to the present system, the slurry ispressurized by moving a punch to effect mechanical compression anddischarge a liquid. Filter cloth are attached to a portion freed theliquid for preparing a molded article. The liquid in the slurry ispassed via the molded article so as to be discharged out of the system.If the pressurization during molding is insufficient, liquid removalfrom the slurry in the vicinity of the punch is insufficient so that anon-solid portion is left. If the pressurization is excessive, the punchis pushed wastefully and the molded article is mechanically compressedso that the packing density of the powders in the molded article islocally increased in the vicinity of the punch with progress in themechanical compression of the molded product. The result is deformationand insufficient dimensional accuracy of a sintered body as the product.The molded product exhibiting considerable fluctuations in density issusceptible to cracking and destruction. From this it follows that thetiming of the end of pressurization needs to be set nearly when thesufficient amount of liquid is removed and all slurry has solidified.The optimum timing of the end of pressurization can be controlled bycontrolling the time or the amount of punch displacement.

FIGS. 7 and 8 illustrates a pressurizing step of the present invention.FIG. 7 schematically shows the relation between pressurizing time andpunch displacement. FIG. 8 schematically shows the molding state of aslurry at each stage of the pressurizing time. A point at which thepacking density of ceramic particles in the slurry becomes uniform inthe pressurizing direction, in other words, a point at which theconcentration gradient in the pressurizing direction of ceramicparticles in the slurry becomes null, is defined as the state in whichall the slurry charged into the cavity has solidified. The pressurizingtime which elapses until solidification is defined at T. The above willbe understood or comparing the unsolidified stage shown at the thirdfrom left to the stage of the solidified slurry shown at the forth fromleft.

According to the present invention, pressurization is caused to proceedfurther continuously for a pre-set time from this stage and is thenstopped in order to take out a molded product from the mold. By carryingout this pressurization for a pre-set time since solidification, themolded product may be safeguarded against damage and lowering indimensional accuracy.

The time control can be achieved by terminating the pressurizationwithin 50% of the pressing time determined from the time at which theslurry in the mold is dehydrated and all slurry turned into a moldedarticle. In other words, pressing of the slurry is completed at a timebetween T and 1.50T, wherein T is the pressing time necessary to removesufficient excess liquid from the slurry in the mold to produce a moldedmass. If the timing of the end of pressurization exceeds 50%, densityfluctuations can occur and the sintered product is deteriorated indimensional stability. In the worst case, cracks are produced.

If no cracks due to density fluctuations are produced, the moldedproduct is abnormally compressed, so that it becomes intimately affixedto the punch or a solvent removing portion resulting in lowered moldrelease properties and crack exfoliation. In addition, since the moldedarticle tends to be expanded (springback) when being taken out of themold, a strong friction operates between the mold and the molded articlethus resulting in the mold injuring the molded article or producingcracks.

Such effect on the molded article is manifested severely incharacteristics of the sintered article thus lowering the dimensionalaccuracy of the sintered article. In addition, mechanical properties ofthe sintered article, such as three-point strength, are loweredsignificantly.

The pressurization and timing is related with the shape of the moldedarticle. That is, the condition is changed depending upon the ratio ofthe size of the article portion thrust by the punch to the size of theproceeding direction of the punch. This ratio (molding length/maximumsize in the cross-section) is termed the aspect ratio. With a moldedarticle having a large aspect ratio, the force of friction actingbetween the molded article and the lateral surface of the mold is solarge that uniform pressure can hardly be applied to the entire moldedarticle. If excess pressure is applied to an article under such state,significant non-uniform pressure is produced in the longitudinaldirection, such that only a small amount of an excess pressure inducesdensity fluctuations or fracture of molded articles. Thus it becomesnecessary to strictly control the pressurization end timing.

The total pressurizing time is up to 1.50 times T, that is in a range offrom T to 1.5T for the aspect ratio of the molded article of 1 or lessand is up to 1.12 times T, that is in a range of from T to 1.12T for theaspect ratio exceeding 1. If the aspect ratio is within this range, thelowering of molded release properties or damages to the molded articledue to friction with the mold may be suppressed by the above-mentionedreason. In addition, the sintered article may be improved in dimensionalaccuracy or strength.

The pressing condition may be implemented by the amount of the punchdisplacement. That is, the pressurization is terminated within the timethe punch is moved distance equal to 17% (0.17 l) of the molding lengthL, as shown in FIG. 8, as measured from a punch position at which excessliquid is removed from the slurry in the mold and the entire slurry hassolidified. In other words, the punch displacement could go beyond apoint at which sufficient excess liquid is removed from the slurry inthe mold and the entire slurry has solidified. The punch displacementstops from a point at which sufficient excess liquid is removed from theslurry in the mold to produce a molded mass is 17% (0.17 L) or less ofthe total molding length (L). If the punch is moved for pressurizationin excess of 17% of the molding length, the molded article can undergodensity fluctuations and, in extreme cases, cracks occur.

In addition, there are occasions where the molded article is injured dueto the lowering of the mold release properties and to friction with themold. The sintered product may also be severely affected in dimensionalaccuracy and mechanical strength, such that, if the aspect ratio exceeds17%l, poor dimensional accuracy or lowering in mechanical properties,such as three-point bending strength, is produced. In addition, as forthe pressurizing condition, the allowable control range of the punchdisplacement is changed depending upon the range of the aspect ratio.That is, when the aspect ratio of the molded product exceeds 1,pressurization ceases within the time the punch is moved a distanceequal to 4% of the molding length from a punch position at which anyexcess solvent is removed from all the slurry in the mold and themolding mass is turned into the molded article. If the punch is movedfor pressurization beyond a point corresponding to 4% of the moldinglength, the molded article undergoes density fluctuations and, in anextreme case, cracking is incurred.

By the reason given hereinabove, the lowering in mold releaseproperties, damages to the molded article due to friction with the mold,poor dimensional accuracy of the sintered product or deterioration inmechanical properties may be incurred.

The pressure molding of the present system is the pressing techniqueequivalent to pressure mechanical compression at an extremely highpressure. Since a correspondingly high mechanical compression acts onthe liquid removing portion, it is desirable to use a material of highmechanical strength; a porous member of metal or ceramics. If a filtersuch as a filter paper and/or a filter cloth is provided on the frontsurface of a slurry contacting portion of a porous member, mold releaseproperties are improved and satisfactory molded products are producedwithout being constrained by the state of the porous member, such aspore size or surface roughness.

With the present system, mechanical compression is applied to the liquidremoving portion. If such compression should occur repeatedly at ahigher pressure of not less than 200 kfg/cm², even a porous member ofmetal or plastics undergoes fatigue to produce surface destruction ordeformation. Thus, it is desirable to use a metal plate having anopening of 0.8 mm or less in diameter in place of the porous member incase of using a filter. Such a metal plate is superior in mechanicaldurability and is not susceptible to fatigue. If the opening in themetal plate exceeds 0.8 mm, the powders may press the filter under themolding pressure to enter the pores to form a protrusions having a shapesimilar to that of opening 1 which is left on the surface of the moldedarticle to affect the surface state of the molded article.

The pore size of the porous member in contact with the slurry needs tobe not more than 20 times the mean particle size of the secondaryparticles of the powders present in the slurry and not less than 0.1 μmby reason of the time of liquid removal and mold release properties. Thesurface roughness of the porous member in contact with the slurry needsto be not more than 0.4 μm Rz by reason of the mold release properties.The surface roughness of the porous member in contact with the slurryneeds to be not more than 0.4 μm Rz by reason of the mold releaseproperties of the molded article. The pore diameter of the filter paperor filter cloth in contact with the slurry needs to be not less than 0.1μm and not more than 20 times the mean particle size of the secondaryparticles of the powders present in the slurry by reason of mold releaseproperties and the time for solvent removal.

With the present system, the punch is moved within a die or mandrel andthe slurry is mechanically compressed as the punch is slid against theinner wall of the die. During slurry compression, slurry effluenceoccurs by reason of the clearance of the sliding portions, such that theslurry falls into shortage and solidification is not completed. Suchleakage is not induced when only press molding the powders as isconventionally done. To prohibit slurry leakage due to such clearance, asealant is provided at the sliding portion. O-rings formed of rubber,resin or metal, for example, may be employed as the sealant. With thepresent system, molding is achieved as the punch is slid against thedie. It is therefore preferable for the die and the punch to be formedof the material which is superior in abrasion resistance and is notsusceptible to fatigue due to repeated sliding of the punch relative tothe die. That is, with the present molding system, it is advantageous touse a die and a punch exhibiting high mechanical strength capable ofwithstanding high pressure and high abrasion resistance. As suchmaterial, a metal material, such as cemented carbide or high-speed steelis preferred.

If the state of the slurry is defined in the system of the presentapplication, it becomes possible to produce a molded product of anintricate shape which cannot be produced with the conventional uniaxialpressurization employing a die and a punch. A product having a shapewith the aspect ratio exceeding 3, a trumpet shape with a changingcross-section or a cylindrical shape can be molded. For producing moldedarticles of such shape, the slurry needs to be flowed uniformly into themold, so that the slurry is preferably of a viscosity not higher than5000 cps.

In addition, in order for the slurry viscosity to be 5000 cps or less,the volume fraction of powders should in general be 60 vol % or less,depending upon the state and types of the starting powders and thesolvent. There is no particular limitation to the powder type, particlesize particle size distribution or solvent type.

The invention will be more clearly understood with reference to thefollowing examples.

EXAMPLE 1

A mixture of powders was made by adding Y₂O₃, Al₂O₃ as assistant agentsto Si₃N₄ powders, having an average diameter of 0.7 μm, then mixing. itin ethylalcohol. Water and binder were added to the mixture. Making useof a nylon ball mill, they were made into a slurry. The powder contentof the slurry was set to be 40 vol. %.

Columns, having a diameter of 20 mm and a height of 20 mm, were moldedout of the slurry. FIGS. 1 and 2 show a process for molding. Mold 3,having holes 5 for dehydration and attached to porous material 7, wasset in mold 2 and the slurry was injected into it. The slurry waspressurized with a metallic punch 1 fitted with a porous punch 6 at itsterminal end. Water in the slurry 8 was discharged by applying pressurethrough porous materials 6, 7. As a result thereof, molded ceramicarticles could be taken out.

A number of molded ceramic articles were made by this molding methodunder various conditions, changing porous materials and moldingpressure. Table 1 shows results of the density of molded articles andmolding time.

TABLE 1 Density Molding of molded Molding Porous pressure articles timeNo. materials (kgf/cm²) (%) (sec.)  1 SUS 316 5 49.8 1000  2 SUS 316 2050.7 250  3 SUS 316 100 61.6 45  4 SUS 316 1000 63.4 12 *5 gypsum 5 48.9850 *6 gypsum 20 gypsum — fractured *7 resin 5 49.7 800 *8 resin 20 50.8200 *9 resin 100 resin — fractured *indicates comparisons

EXAMPLE 2

As starting powders, Si₃N₄ (90-95 wt %) with mean primary particle sizeof 0.2 μm and mean secondary particle size of 0.5 μm, as main component,Y₂O₃ (mean primary particle size, 0.5 μm) and Al₂O₃ (mean primaryparticle size, 0.3 μm), as additives, are employed. The starting powderswere mixed in a ball mill with distilled water and admixed with a binderand thus a slurry having a volume concentration of 42 vol % of thepowders was prepared.

The mean secondary particle size was 0.53 μm (due to the effect of theadditive added to 0.5 μm of the main component). Using this slurry, adisc 50 mm in diameter and about 6 mm in thickness was molded. In thiscase, the aspect ratio was 0.12.

Molding was carried out in the same way as in Example 1, that is withthe method shown in FIG. 2.

As a dehydrating part, a SUS plate having a through-hole, with a filterpaper and a filter cloth placed ahead of it, was used.

In the present Example, the punch and the die were formed of cementedcarbide and the mold was formed of high-speed steel. Molding was carriedout under a high pressure of 300 kgf/cm².

The pressurizing conditions were controlled by the molding time or theamount of punch displacement (aspect ratio, not more than 1). With theuse of the slurry quantity which will give a molding thickness of 6 mmfor molding, the optimum pressurization end timing was set by timecontrol (within a time (sec) between T and 1.50T, wherein T was thepressurization time necessary to dehydrate all the slurry in the mold togive a molded article). and punch displacement control (17% or less ofthe molding length as from the position at which all the slurry in themold is dehydrated to give a molded article). Other molding conditionswere also varied in many ways.

The relation of mold release properties and the molding state andvarious conditions are as shown in Table 2. The produced molded articlewas divided in the cross-section in five equal parts and the density ofthe molded product was measured to calculate density fluctuations in themolded article from the maximum and minimum values.

A sintered article was prepared from this molded article and sliced intotest pieces for bending test (3×4×35 mm) in order to measure thethree-point strength (n=30).

TABLE 2 molding conditions mean filter mean pore molding punch punchpore size filter size of SUS times position displacement sealing die No.Claims method filter (μm) material (μm) (sec) (mm) (mm) O-ring material 1 Comp. 12, 9 3 present 5.0 filter through 20x 6.1x −1.0 presentcemented Ex. paper + holes carbide cloth 700  2 Ex. 12, 9 3 present 5.0filter through 26 6.0 0.0 present cemented paper + holes carbide cloth700  3 Ex. 12, 9 3 present 5.0 filter through 32 5.4 0.6 presentcemented paper + holes carbide cloth 700  4 Comp. 12, 9 3 present 5.0filter through 41x 4.9 1.1 present cemented Ex. paper + holes carbidecloth 700  5 Ex. 9 3 present 5.0 filter through 28 6.0 0.0 presentcemented paper + holes carbide cloth 500x  6 Comp. 9 3 present 5.0filter through 26 6.0 0.0 present cemented Ex. paper + holes carbidecloth 900x  7 Comp. 9 3 present 5.0 filter through 26 6.0 0.0 presentcemented Ex. paper + holes carbide cloth 1100x x mark denotes not beingclaimed results of molding density of mold density bending molded bodyrelease defects of fluctuations strength No. claims (g/cm²) molded stateproperties molded articles (%) (kg/mm²) 1 Comp. 12, 9 62.5 dehydration —non-solidified 1.8 — Ex. incomplete 2 Ex. 12, 9 63.4 good good good 0.3138 3 Ex. 12, 9 63.6 good good good 0.4 141 4 Comp. 12, 9 64.3 good goodcracking 1.9 82 Ex. 5 Ex. 9 63.6 good good good 0.3 137 6 Comp. 9 63.0good good protruded height 0.8 140 Ex. of molded product 0.8 mm 7 Comp.9 63.1 good good protruded height 0.9 141 Ex. of molded product 0.95 mm

EXAMPLE 3

As starting powders, Al₂O₃ (mean primary particle size, 0.2 μm; meansecondary particle size, 0.4 μm) was used. The starting powders weremixed in a ball mill with distilled water and admixed with a binder toproduce a slurry having a volume concentration of the powders of 50 vol%.

Using this slurry, a column (cylinder) 15 mm in diameter and about 60 mmin length was molded by the same molding method as that of Examples 1and 2 shown in FIG. 2. In this case, the aspect ratio was 4.

As a dehydrating part, a plate of SUS with a through-hole, having afilter paper and/or a filter cloth ahead of it, was used.

In the present Example, the punch and the die were formed of cementedcarbide and the mold was formed of high-speed steel. Molding was carriedout under a high pressure of 500 kgf/cm.

The pressurization during molding was controlled by controlling themolding time or and/or the amount of punch displacement (aspect ratio, 1or high).

The quantity of slurry which will give the molding length of 60 mm wasused for molding. The optimum pressurization end timing was by timecontrol (within a time (sec) between T and 1.12T, wherein T was thepressurization time necessary to dehydrate all the slurry in the mold tofive a molded product) and by punch displacement control (within 4% ofthe molding length as from the position when all the slurry in themolded is dehydrated and turned into slurry).

Based on the range in this setting method, molding was carried out usingvariable molding time lengths and punch displacement amounts.

Filter pore sizes, as other conditions, were changed in many ways.

These various conditions, state of molding and mold release propertiesare as shown in Table 3. The molded article was divided into five equalparts along its length and the density of the molded article wasmeasured in order to measure density fluctuations in the molded articlefrom the maximum and minimum values.

TABLE 3 molding conditions mean filter mean pore molding punch punchpore size filter size of SUS times position displacement sealing die No.Claims method filter (μm) material (μm) (sec) (mm) (mm) O-ring material 9 Comp. 3, 4, 9 3 present 5.0 filter through 116x 51.8x −1.8 presentcemented Ex. paper + holes carbide cloth 700 10 Ex. 3, 4, 9 3 present5.0 filter through 124 60.0 0.0 present cemented paper + holes carbidecloth 700 11 Comp. 3, 4, 9 3 present 5.0 filter through 132 47.2x 2.8present cemented Ex. paper + holes carbide cloth 700 12 Comp. 3, 4, 9 3present 5.0 filter through 128 59.8 1.2 present cemented Ex. paper +holes carbide cloth 700 29 Comp. 10 3 present 27.7x filter through 12859.8 1.2 present cemented Ex. cloth holes carbide 700 Comp. 10 3 present15.2x filter through 127 59.8 1.1 present cemented Ex. cloth holescarbide 700 x mark denotes not being claimed results of molding densityof mold density bending molded body release defects of fluctuationsstrength No. claims (g/cm²) molded state properties molded articles (%)(kg/mm²)  9 Comp. 3, 4, 9 71.3 dehydration — non-solidified 1.7 Ex.incomplete 10 Ex. 3, 4, 9 74.0 good good good 0.3 11 Comp. 3, 4, 9 74.1good good good 0.3 Ex. 12 Comp. 10 74.8 good good cracking 3.9 Ex. 29Comp. 10 59.5 slurry exrded — non-solidified 2.5 Ex. Comp. 10 60.1slurry extrded — non-solidified 2.3 Ex.

EXAMPLE 4

The slurry was made by mixing together Al₂O₃ powders having averagediameter of 1 μm, distilled water using a ball mill and admixing with abinder. The powder content of the slurry was set to be 53 vol. %.Columns, having a diameter of 20 mm and a height of 20 mm, were moldedout of the slurry using the molding method illustrated in FIG. 1.

Stainless steel, having different diameters of pores and surfaceroughness, was used as porous materials, and the molding pressureapplied in this case was 200 and 800 kgf/cm².

The diameter of pores on the surface was determined by taking theaverage through observations under a microscope. Table 4 shows resultsof the density of molded articles, molding time and mold releaseproperties with respect to the porous materials, under variousconditions.

TABLE 4 Average diameter Surface Density of pores roughness of Moldingof porous of porous molded Molding pressure materials materials articlestime Status of No. (kgf/cm²) (μm) μm in Rz (%) (sec.) mold release 1 2000.07 0.2 — not — stiffened in 300 2 200 0.53 0.2 67.7 79 satisfactory 3200 0.53 0.5 67.4 77 peeled off 4 200 4.8 0.3 68.8 53 satisfactory 5 2004.8 0.6 68.7 54 peeled off 6 200 21.3 0.3 67.6 42 peeled off 7 800 0.070.2 — not — stiffened in 300 8 800 0.53 0.2 69.4 69 satisfactory 9 8000.53 0.5 69.2 68 peeled off 10 800 4.8 0.3 70.7 47 satisfactory 11 8004.8 0.6 70.5 45 peeled off 12 800 21.3 0.3 70.0 35 peeled off

EXAMPLE 5

Y₂O₃, Al₂O₃ as assistant agents were added to Si₃N₄ powders having anaverage diameter of 0.5 μm. Then they were mixed in distilled water bymaking use of a ball mill. Some binder was added to the mixture and theywere mixed further to make a slurry. The powder content of the slurrywas 42 vol. %. Measurements of the particle size distribution indicatedthe mean particle diameter to be 0.53 μm.

Disks, having a diameter of 40 mm and a thickness of 5 mm, were moldedout of the slurry.

FIG. 2 shows a process for molding. Conditions were changed variously;diameters of pores in the porous stainless steel, surface roughness anddiameters of pores in the filter. Casting without filter and casting asillustrated in FIG. 3 were practiced for comparison. And moldingpressure applied in this case was 300 kgf/cm².

Table 5 and 6 show how some conditions affect the molded ceramicarticles and statuses of mold releasing. It may be seen from theseTables that satisfactory cast articles could be produced in accordancewith the present invention.

TABLE 5 Average Average diameter diameter of pores Filter of pores ofmetallic present of filter Materials materials No. Process or none (μm)of filter (μm) 1 FIG. 2 none — — 20.1 2 FIG. 2 none — — 72.2 3 FIG. 2none — — 8.2 4 FIG. 2 filter 0.05 resin film 20.1 5 FIG. 2 filter 0.05resin film 72.2 6 FIG. 2 filter 0.4 filter paper 20.1 7 FIG. 2 filter0.4 filter paper 72.2 8 FIG. 2 filter 0.4 filter paper 8.2 9 FIG. 2filter 5.0 filter paper 20.1 10 FIG. 2 filter 5.0 filter paper 72.2 11FIG. 2 filter 5.0 filter paper 8.2 12 FIG. 2 filter 27.7 filter cloth20.1 13 FIG. 2 filter 27.7 filter cloth 72.2 14 FIG. 2 filter 27.7filter cloth 8.2 diameter of holes for the hydration 15 FIG. 3 filter5.0 filter paper 1200 plus filter cloth 16 FIG. 3 filter 5.0 filterpaper 2000 plus filter cloth 17 FIG. 3 filter 5.0 filter paper 2400 plusfilter cloth

TABLE 6 Surface roughness Density of porous of molded Molding Status ofmolded metallic materials articles time articles and No. μm in R2 (%)(sec.) mold releasing 1 0.2 62.3 24 peeled off on mold releasing 2 0.363.4 25 peeled off on mold releasing 3 1.0 62.2 30 peeled off on moldreleasing 4 0.2 59.9 359 took long time in dehydration 5 0.3 59.8 354took long time in dehydration 6 0.2 64.1 21 satisfactory 7 0.3 64.3 22satisfactory 8 1.0 64.0 21 satisfactory 9 0.2 63.9 19 satisfactory 100.3 64.2 20 satisfactory 11 1.0 64.1 18 satisfactory 12 0.2 59.9 18slurry permeated the filter 13 0.3 59.9 18 slurry permeated the filter14 1.0 59.7 19 slurry permeated the filter 15 — 63.2 20 protrusions(about 1 mmφ) formed on the molded articles 16 — 63.1 19 protrusions(about 1.7 mmφ) formed on the molded articles 17 — 63.2 20 protrusions(about 2 mmφ) formed on the molded articles

EXAMPLE 6

A mixture of powders was made by adding Y₂ _(O) ₃, Al₂O₃ as assistantagents to Si₃N₄ powder having an average diameter of 0.8 μm, mixing it,in ethylalcohol and by drying it. Deionized water and a binder wereadded to the mixture. Making use of a nylon ball mill, they were madeinto slurry. The powder content of the slurry was set to be 40 vol. %.

Columns having a diameter of 10 mm and a height of 25 mm, were moldedout of the slurry. The process for molding was the same as Example 1, asshown in FIG. 1. Molded ceramic articles were made under variousconditions, using different kinds of porous materials and changingmolding pressure in the process. Table 7 shows results of the density ofmolded articles and molding time.

TABLE 7 Density Molding of molded Molding Porous pressure articles timeNo. materials (kgf/cm²) (%) (sec.)  1 Al₂O₃ 2 50.5 840  2 Al₂O₃ 20 51.3220  3 Al₂O₃ 200 63.5 32  4 Al₂O₃ 950 64.7 9 *5 qypsum 2 49.2 720 *6gypsum 20 gypsum — fractured *7 resin 2 49.9 720 *8 resin 20 51.2 200 *9resin 200 resin — fractured *indicates comparisons

EXAMPLE 7

Using a ball mill, containing balls of Al₂O₃ the slurry was made bymixing together Al₂O₃, powder having an average diameter of 1 μm,distilled water and some binder. The powder content of the slurry was tobe 53 vol. %. Columns, having a diameter of 10 mm and a height of 20 mm,were molded out of the slurry. The process for molding was the same asExample 1, as shown in FIG. 1.

Al₂O₃, with different diameters of pores and surface roughness, wasused. The cavity rate of the Al₂O₃ was 38 vol. %, and the moldingpressure applied in this case was 200 and 800 kgf/cm².

The surface pore diameters were measured by observation with amicroscope (SEM and optical microscope) and determined in terms of amean value. Table 8 shows results of density of molded articles, moldingtime and mold release properties with respect to the porous materialsunder various conditions.

TABLE 8 Average diameter Surface of pores roughness Density Molding ofporous of porous of molded Molding Status of molded pressure materialsmaterials articles time articles and No. (kgf/cm²) (μm) μm in R2 (%)(sec.) mold releasing 1 200 0.07 0.2 — not — stiffened in 300 2 200 0.710.2 68.2 72 satisfactory 3 200 0.71 0.7 67.9 69 peeled off 4 200 8.7 0.369.3 50 satisfactory 5 200 8.7 0.7 69.2 51 peeled off 6 200 24.4 0.368.1 35 peeled off 7 800 0.07 0.2 — not — stiffened in 300 8 800 0.710.2 69.5 65 satisfactory 9 800 0.71 0.5 69.9 62 peeled off 10 800 0.710.7 69.7 64 peeled off 11 800 8.7 0.3 71.2 37 satisfactory 12 800 8.70.5 71.0 36 peeled off 13 800 8.7 0.7 71.0 33 peeled off 14 800 24.4 0.370.5 28 peeled off

EXAMPLE 8

As starting powders, Si₃N₄ (90 to 95 wt %) with mean primary particlesize of 0.5 μm and mean secondary particle size of 1.0 μm), as a maincomponent, Y₂ _(O) ₃ (mean primary particle size of 0.5 μm) and Al₂O₃(mean primary particle size of 0.3 μm), as additives, were used. Thestarting powders were mixed in a ball mill with distilled water andadmixed with a binder in order to prepare slurries with volumeconcentrations of 42, 52 and 62 vol %.

The mean secondary particle sizes of the slurries were 1.3, 1.8 and 2.4μm.

Using these slurries, molded articles in the form of circular trumpetshaving a length of 100 mm and diameters of cross-sectional surfaces of20 mm and 10 mm on both ends were prepared (FIG. 6).

Using slurry quantities which will give 100 mm of the molding length,the optimum pressurization and timing of the three slurries werecontrolled so as to be within 4% of the molding length (100 mm) as fromthe position at which all the slurry in the mold is dehydrated andturned into molded article.

The various conditions and the states of the molded article were asshown in Table 9. Density of molded product was measured only of thecolumnar portion of 10 mm diameter, which were severed and taken out.

TABLE 9 molding condition punch slurry mean pore displacement moldedarticle volume size of porous punch position (distance density of conc.viscosity ceramic article (molding length) from 100 mm) molded articledefects in No. (vol %) (cps.) (μm) (mm) (mm) (g/cm³) molded articles  142 1320 4.2 99 1 56.1 good  2 42 1320 13.5 98 2 56.6 good  3 42 132019.7 100 0 56.8 good  4 52 2780 4.2 100 0 58.4 good  5 52 2780 13.5 98 258.2 good  6 52 2780 19.7 98 2 58.4 good *7 62 6500 4.2 99 1 64.1fracture at 10 mm diameter portion *8 62 6500 13.5 98 2 63.1 fracture at10 mm diameter portion *9 62 6500 19.7 98 2 63.3 fracture at 10 mmdiameter portion *indicates comparisons

EXAMPLE 9

Y₂O₃ and Al₂O₃ as assistant agents were added to Si₃N₄ powder having anaverage diameter of 0.5 μm. Then they were mixed in distilled water bymaking use of a ball mill. Some binder was added to the mixture and theywere mixed further to make the slurry. The powder content of the slurrywas 42 vol. %. The average diameter was indicated to be 0.53 μm bymeasurement of the size distribution.

Disks, having a diameter of 40 mm and a thickness of 5 mm, were moldedout of the slurry through the process for molding as shown in FIG. 2.

The porous material was Al₂O₃. And other conditions were changedvariously; diameters and surface roughness of the porous materials anddiameters of pores in the filter. Molding without the filter and moldingas illustrated in FIG. 3 were also performed for comparison. And moldingpressure applied in this case was 300 kgf/cm². Table 10 and 11 show howsome conditions affect the molded ceramic articles and its status ofmolded articles and mold releasing. They also show that a satisfactorymolded ceramic articles could be produced in accordance with the presentinvention.

TABLE 10 Average Average diameter of diameter pores of Filter of poresceramics present of filter Materials materials No. Process or none (μm)of filter (μm) 1 FIG. 2 none — — 24.4 2 FIG. 2 none — — 72.2 3 FIG. 2none — — 8.7 4 FIG. 2 filter 0.08 resin film 24.4 5 FIG. 2 filter 0.08resin film 72.2 6 FIG. 2 filter 0.6 filter paper 24.4 7 FIG. 2 filter0.6 filter paper 72.2 8 FIG. 2 filter 0.6 filter paper 8.7 9 FIG. 2filter 0.6 filter paper 72.2 10 FIG. 2 filter 4.0 filter paper 24.4 11FIG. 2 filter 4.0 filter paper 72.2 12 FIG. 2 filter 4.0 filter paper8.7 13 FIG. 2 filter 4.0 filter paper 72.2 14 FIG. 2 filter 23.2 filtercloth 24.4 15 FIG. 2 filter 23.2 filter cloth 72.2 16 FIG. 2 filter 23.2filter cloth 8.7 17 FIG. 3 filter 4.0 filter paper diameter of plusfilter holes for the cloth hydration 1400 18 FIG. 3 filter 4.0 filterpaper 2000 plus filter cloth 19 FIG. 3 filter 4.0 filter paper 2500 plusfilter cloth

TABLE 11 Surface rough- ness of porous Status ceramics Density of ofmolded materials molded articles Molding time articles and No. (Rz) (%)(sec.) mold releasing  1 0.2 62.8 25 peeled off in releasing  2 0.3 63.924 peeled off in releasing  3 1.0 62.7 30 peeled off in releasing  4 0.260.4 370 took long time in dehydration  5 0.3 60.2 368 took long time indehydration  6 0.2 64.6 21 satisfactory  7 0.3 64.8 19 satisfactory  81.0 64.5 21 satisfactory  9 5.0 64.2 19 satisfactory 10 0.2 64.4 22satisfactory 11 0.3 64.7 18 satisfactory 12 1.0 64.6 23 satisfactory 135.0 64.6 18 satisfactory 14 0.2 60.4 19 slurry permeat- ed the filter 150.3 60.4 19 slurry permeat- ed the filter 16 1.0 60.2 20 slurry permeat-ed the filter 17 — 63.7 19 protrusions (about 1.2 mmφ) formed on themolded articles 18 — 63.6 19 protrusions (about 1.7 mmφ) formed on themolded articles 19 — 63.7 18 protrusions (about 2.3 mmφ) formed on themolded articles

EXAMPLE 10

Using the same slurry as that of Example 6, columns, having a diameterof 10 mm and a length of 50 mm, were molded out of the slurry in thesame way as shown in FIG. 2. Al₂O₃, having an average surface porediameter of 24.4 μm and a surface roughness of 1.0 z, was used as porousceramics. A filter paper having an average diameter of 1.0 μm, was usedas filter. A vacuum pump (rotary) was connected with the dehydrationholes in the upper mold 2, through a trap for catching water, and thepump sucked the water forcibly. Molding pressure applied in this casewas 500 kgf/cm² and the sucking started simultaneously with thepressing. Table 12 shows the time and the states of molded ceramics.

TABLE 12 Forcible Pressing Density sucking time of molded Status ofmolded No. or none (sec.) articles (%) articles 1 forcible 15 64.3satisfactory sucking 2 forcible 45 64.5 satisfactory sucking 3 forcible120 64.2 satisfactory sucking 4 none 15 64.2 not wholly (only forstiffened stiffened, slurry portion) partially left 5 none 45 64.4satisfactory 6 none 120 64.3 satisfactory

As described above, by the invention, molded ceramic articles having ahigh density and a smoothed surface, could be surely produced in arelatively short time.

In order to further demonstrate the advantages of the inventive method,comparative tests were conducted with various combinations of filtersand molds as shown in Table 13. In performing this comparison, thefollowing parameters were maintained.

To obtain a molding time necessary for industrial application a presetnumber of through-holes per unit area of a metal plate is required. Tothis end, it is preferable to provide through-holes at a pitch of notmore than 10 mm in the metal plate. If the through-hole pitch exceeds 10mm, removal of the solvent in the slurry becomes time-consuming which isindustrially disadvantageous.

As starting powders, Si₃N₄ (90-95 wt %) with the mean primary particlesize of 0.2 μm and the mean secondary particle size of 0.5 μm, as a maincomponent, and Y₂O₃, with the mean primary particle size of 0.5 μm andAl₂O₃ with the mean primary particle size of 0.3 μm, as additives, wereused. The starting powders were mixed in a ball mill with distilledwater and added to a binder to give a slurry with a volumetricconcentration of the powders of 42 vol %.

The mean secondary particle size of the slurry was 0.53 μm (with theparticle size of the main component of 0.5 μm which is raised to 0.53 μmunder the effect of the additives). Using this slurry, a cylinder havinga diameter of 10 mm and a length of 50 mm was produced. In this caseaspect ratio is 5 (50 mm/10 mm).

The concept of the molding method is shown in FIG. 2. The moldingequipment basically included punch 1, die 2, mold 3 and a dehydrator.Molding was performed by pressurizing the slurry within the die with apunch, as in the case of press molding. The moisture in the slurry wasremoved from the system via the dehydrator. The dehydrated slurry wassolidified to give a molded article. Pressurization was performed evenlywith a punch having the same cross-section as the product cross-section.Since the slurry intruded into the space between the punch and theslurry (clearance), a sealant such as an O-ring was provided therein.

The dehydrator included (i) a porous member formed of SUS (method 2),(ii) a porous member of SUS having a filter such as filter paper orfilter cloth on the front surface, or (iii) an SUS plate havingthrough-hole and a filter such as a filter paper or a filter cloth onits front side (method 3).

With (ii) and (iii), the slurry is not in direct contact with the porousmember. Since the punch, die and the mold are formed of metal, such ascemented carbide, as in the case of customary press molding, it ispossible to use a higher molding pressure than in the conventional casewherein the device in its entirety is formed by a porous member. In theembodiment illustrated, the punch and the cavity were formed of cementedcarbide, while the mold was formed of high speed steel. The molding wascarried out at an elevated pressure of 300 kgf/mm². For comparison,molding was also carried out in accordance with (method 3)-1 in which nosealant was used in the clearance between the punch and the die, and(method 3)-2 in which the die was formed of a resin used in aconventional casting mold.

The pressurizing conditions during molding were controlled by themolding time or punch displacement. The molding conditions weredetermined as later described. If the pressurization is insufficient,slurry dehydration in the vicinity of the punch becomes insufficient anda non-solid portion is left. Conversely, if excess pressurization ismade, the punch is moved excessively to mechanically compact the moldedproduct, thus locally increasing the powder packing density in themolded product in the vicinity of the punch. The result is poordimensional accuracy and deformation of the sintered product. If themolded article exhibits significant density fluctuations in the moldedarticle, the article undergoes cracks or destruction. From this itfollows that the pressurization end timing must be within a certainrange as from the time point when a present amount of the moisture inthe slurry in the mold is dehydrated and the slurry in its entirety hasbeen molded.

The slurry was used in a quantity which gave a molding length of 50 mm.The optimum pressurization end timing was set in accordance with one ofthe following two methods:

(i) timing control

within a time (sec) between T and 1.2T, wherein T was the pressurizationtime necessary to dehydrate all the slurry in the mold to give a moldedproduct)

(ii) control of punch displacement

within 4% of the molding length as from the position at which thetotality of the slurry within the mold is dehydrated to give a moldedarticle.

Based on the above range, molding was carried out in accordance with avariety of molding time durations and punch displacements. The pore sizeand surface roughness of the porous member and the filter pore size, asother molding conditions, were also varied.

The relation of the various conditions, molding state and mold releaseproperties were as shown in Table 13 and 14. The produced moldedproducts were divided in five equal portions and the density of the moldproducts was measured. Density fluctuations in the molded products werecalculated from the maximum and minimum density values.

TABLE 13 molding conditions mean filter mean pore molding punch punchpore size filter size of SUS times position displacement sealing die No.Claims method filter (μm) material (μm) (sec) (mm) (mm) O-ring material 1 Comp. 3 1 absent — —  8.2 0.2  20x 51.0x present cemented Ex. carbide 2 Ex. 3 1 absent — —  8.2 0.2  25 50.0 present cemented carbide  3 Ex.3 1 absent — —  8.2 0.2  27 48.9 present cemented carbide  4 Comp. 3 1absent — —  8.2 0.2  45x 47.5x present cemented Ex. carbide  5 Comp. 4 2present  0.4 filter 72.2 0.3  16x 51.1x present cemented Ex. papercarbide  6 Ex. 4 2 present  0.4 filter 72.2 0.3  20 50.0 presentcemented paper carbide  7 Ex. 4 2 present  0.4 filter 72.2 0.3  21 49.5present cemented paper carbide  8 Comp. 4 2 present  0.4 filter 72.2 0.3 23x 47.7x present cemented Ex. paper carbide  9 Comp. 5 3 present  5.0filter through —  16x 51.8x present cemented Ex. paper + holes carbidecloth 700*² 10 Ex. 5 3 present  5.0 filter through —  20 50.0 presentcemented paper + holes carbide cloth 700*² 11 Ex. 5 3 present  5.0filter through —  21 49.4 present cemented paper + holes carbide cloth700*² 12 Comp. 5 3 present  5.0 filter through —  24x 47.0x presentcemented Ex. paper + holes carbide cloth 700*² (10) Ex. 3 present  5.0filter through —  20 50.0 present cemented paper + holes carbide cloth700* 13 Comp. 5 3 present  5.0 filter through —  20 50.0 presentcemented Ex. paper + holes carbide cloth 1200x 14 Comp. 5 3 present  5.0filter through —  19 50.0 present cemented Ex. paper + holes carbidecloth 2000x 15 Comp. 5 3 present  5.0 filter through —  20 50.0 presentcemented Ex. paper + holes carbide cloth 2400x (2) Ex. 1 absent — —  8.20.2  25 50.0 present cemented carbide 16 Comp. 6 1 absent — — 20.1x 0.2 25 50.0 present cemented Ex. carbide 17 Comp. 6 1 absent — — 72.2x 0.2 25 50.0 present cemented Ex. carbide 16 Comp. 7 1 absent — —  8.2  2550.0 present cemented Ex. carbide 19 Comp. 8 2 present  0.05x resin 20.10.2 359 50.0 present cemented Ex. film carbide 20 Comp. 8 2 present 0.05x resin 72.2 0.3 354 50.0 present cemented Ex. film carbide 21 Ex.8 2 present  0.4 filter 20.1 0.2  20 50.0 present cemented paper carbide(6) Ex. 3 present  0.4 filter 72.2 0.3  20 50.0 present cemented papercarbide 22 Ex. 8 2 present  0.4 filter 8.2 1.0  21 50.0 present cementedpaper carbide 23 Ex. 8 2 present  5.0 filter 20.1 0.2  19 50.0 presentcemented paper carbide 24 Ex. 8 2 present  5.0 filter 72.2 0.3  20 50.0present cemented paper carbide 25 Ex. 8 2 present  5.0 filter 8.2 1.0 18 50.0 present cemented paper carbide 26 Comp. 8 2 present 27.7xfilter 20.1 0.2  18 50.0 present cemented Ex. cloth carbide 27 Comp. 8 2present 27.7x filter 72.2 0.3  18 50.0 present cemented Ex. clothcarbide 28 Comp. 8 2 present 27.7x filter 8.2 1.0  19 50.0 presentcemented Ex. cloth carbide (10) Ex. 3 present  5.0 filter through —  2050.0 present cemented paper + holes carbide cloth 700 29 Comp. 8 3present 27.7x filter through —  18 50.0 present cemented Ex. cloth holescarbide 700 (10) Ex. 3 present  5.0 filter through —  20 50.0 presentcemented paper + holes carbide cloth 700 30 Comp. 9 3 present  5.0filter through —  20 50.0 absent x cemented Ex. paper + holes carbide700 31 Comp. 10 3 present  5.0 filter through — — — — resin Ex. paper +holes cloth 700 32 Comp. 11 3 present  5.0 filter through — 120 50.0present cemented Ex. paper + holes carbide cloth (pitch 12)x *¹:distance between punch surface and filter surface (when it is 50, allthe slurry solidifies) *²: through-hole pitch is 5 mm unless specified xmark denotes not being claimed

TABLE 14 results of molding density of mold density molded body releasedefects of fluctuations No. (g/cm²) molded state properties moldedarticles (%)  1 Comp. 63.5 dehydration — non-solidified 1.8 Ex.incomplete  2 Ex. 63.9 good good good 0.3  3 Ex. 64.0 good good good 0.3 4 Comp. 64.5 good good cracking 2.9 Ex.  5 Comp. 64.1 dehydration —non-solidified 1.8 Ex. incomplete  6 Ex. 64.3 good good good 0.4  7 Ex.64.4 good good good 0.4  8 Comp. 64.9 good good cracking 2.7 Ex.  9Comp. 63.2 dehydration — non-solidified 1.7 Ex. incomplete 10 Ex. 64.0good good good 0.3 11 Ex. 64.1 good good good 0.3 12 Comp. 64.4 goodgood cracking 2.9 Ex. (10) Ex. 64.0 good good good 0.3 13 Comp. 63.2good good protrusions 0.7 Ex. 1 mmφ 14 Comp. 63.1 good good protrusions0.8 Ex. 1.7 mmφ 15 Comp. 63.2 good good protrusions 0.7 Ex. 2 mmφ (2)Ex. 63.9 good good good 0.3 16 Comp. 62.3 good x peeling of 0.5 Ex.molded product 17 Comp. 63.4 good x peeling of 0.5 Ex. molded product 18Comp. 62.2 good x peeling of 0.4 Ex. molded product 19 Comp. 59.9difficulties good good 0.8 Ex. in hydration, prolonged 20 Comp. 59.8difficulties good good 0.9 Ex. in hydration, prolonged 21 Ex. 64.1 goodgood good 0.3 (6) Ex. 64.3 good good good 0.4 22 Ex. 64.0 good good good0.3 23 Ex. 63.9 good good good 0.3 24 Ex. 64.2 good good good 0.2 25 Ex.64.1 good good good 0.2 26 Comp. 59.9 slurry exrded x peeling of 1.8 Ex.into filter molded product cloth 27 Comp. 59.9 slurry exrded x peelingof 2.0 Ex. into filter molded product cloth 29 Comp. 59.7 slurry exrdedx peeling of 1.9 Ex. into filter molded product cloth (10) Ex. 64.0 goodgood good 0.3 29 Comp. 59.5 slurry exrded — non-solidified 2.1 (10) Ex.64.0 good good good 0.3 30 Comp. 59.7 slurry good non-solidified 2.3leakage 31 Comp. — die destroyed — — — Ex. 32 Comp. 63.7 difficultiesgood good 0.9 Ex. in dehydration

By meeting the inventive molding, and simultaneously selecting thepressure and the mold of the type elucidated in the presentspecification, density fluctuations of the molded article are furtherreduced. In addition, such defects as the surface roughening phenomenon,cracking and/or surface exfoliation may be eliminated. As for a moldedarticle having a smaller length to diameter (L/D) ratio, it is possibleto utilize a broader range of the slurry conditions (secondary particlesize distribution and density) than if only the mold of the typeelucidated in the present specification is used. As for the L/D ratio,the boundary would be approximately equal to 1 (L/D≈1).

By further limiting the slurry conditions, that is the secondaryparticle size distribution and density, it becomes possible to obtain aproduct with a larger L/D ratio with a higher quality and at a higherspeed. The above discussions may be summarized as shown in the followingTable 15.

TABLE 15 Claim Operation and effect Summary of the Examples Contents of(a) As compared to the case of By using a ceramic or and (b) above b)only, density fluctuations metal mold, capable of are reduced anddefects producing slurries with may be eliminated. With certain broadranges of the case of b) only, a Si₃N₄, Al₂O₃, ZrO₂ or SiC homogeneousceramic and withstanding high sintered article with high pressure asshown in the dimension accuracy and present specification, or by highstrength may be simultaneously using a produced. filter shown in thepresent As compared to the case of specification, the effects b) only,it becomes possible given on the left side may to mold an article with abe achieved. high L/D ratio under broader slurry conditions. Molding maybe made at a high pressure and at a high speed. Contents of d) Byfurther limiting the By selecting slurry slurry conditions, articles ofconditions and using the more intricate shape can be above material, andby produced, in addition to the combining the above above effects.various mold types and molds of the specific shape, the effects shown onthe left are produced.

We claim:
 1. A wet press molding method in which a slurry of ceramics ischarged into a cavity and uniaxially pressed by a punch to remove anexcess liquid from a portion facing the punch to effect molding, whereinthe improvement comprising completing pressing the slurry when punchdisplacement from a punch position corresponding to discharge of theexcess liquid from the entire slurry in the mold to produce a moldedmass is less than 17% of a total molding length, L.
 2. The method ofclaim 1 wherein a porous member of ceramics or metal is employed at asolvent removing portion.
 3. The method of claim 2 wherein a mean porediameter of the porous member of metal or ceramics in contact with theslurry is not more than 20 times a mean particle size of the secondaryparticles of a powder present in the slurry.
 4. The method of claim 2wherein a surface of the porous member of ceramics or metal in contactwith the slurry has a surface roughness of not higher than 0.4 μm Rzdefined by JIS B
 0601. 5. The method of claim 1 wherein a porous memberof ceramics or metal having a filter paper and/or a filter cloth on itsfront surface is employed at a solvent removing portion.
 6. The methodof claim 5 wherein said filter paper and/or filter cloth has a mean porediameter of not less than 0.1 μm and not more than 20 times the meanparticle size of secondary particles of a powder which are agglomeratedparticles of primary particles of the powder in the slurry to be molded.7. The method of claim 1 wherein a metal plate having through holes withdiameters of 0.8 mm or less and a filter paper arid/or a filter cloth onits front surface are employed at a solvent removing portion.
 8. Themethod of claim 7 wherein said filter paper and/or filter cloth has amean pore diameter of not less than 0.1 μm and not more than 20 timesthe mean particle size of secondary particles of a powder which areagglomerated particles of primary particles of the powder in the slurryto be molded.
 9. The method of claim 7 wherein the through-holes areprovided at a pitch 10 mm or less.
 10. The method of claim 1 wherein asealing member is provided at a cavity-punch slide portion.
 11. Themethod of claim 1 wherein an abrasion resistant metal material is usedto from the cavity and punch.
 12. The method of claim 1 wherein a porousmember of ceramics or metal having a filter paper and/or a filter clothon its front surface is employed at a liquid removing portion.
 13. Themethod of claim 1 wherein a metal plate having through-holes withdiameters of 0.8 mm or less and a filter paper and/or a filter cloth onits front surface are employed at a liquid removing portion.
 14. Themethod of claim 1 wherein a viscosity of the slurry is less than 5000cps.
 15. The method of claim 1 wherein a volume fraction of powders inthe slurry is less than 60 vol %.
 16. A wet press molding method iswhich a ceramics slurry is charged into a cavity and uniaxially pressedby a punch to remove an excess liquid from a portion facing the punch toeffect molding, and in which the aspect ratio (molding length/sectionsize ratio) of the molding article is more than 1, wherein theimprovement comprising completing pressing the slurry when punchdisplacement from a punch position corresponding to discharge of excessliquid from the entire slurry in the mold to produce a molded mass isless than 4% of a total molding length, L.
 17. The method of claim 16wherein a porous member of ceramics or metal is employed at a solventremoving portion.
 18. The method of claim 16 wherein porous member orceramics or metal having a filter paper and/or a filter cloth on itsfront surface is employed at a liquid removing portion.
 19. The methodof claim 16 wherein a metal plate having through-holes with diameters of0.8 mm or less and a filter paper and/or a filter cloth on its frontsurface are employed at a liquid removing portion.
 20. The method ofclaim 16 wherein a sealing member is provided at a cavity-punch slideportion.
 21. The method of claim 16 wherein an abrasion resistant metalis used to form the cavity and punch.
 22. The method of claim 16 whereina viscosity of the slurry is less than 5000 cps.
 23. The method of claim16 wherein a volume fraction of powders in the slurry is less than 60vol %.